Screwdriver with adjustable device to limit transmitted torque

ABSTRACT

A screwing-tool with a device to adjust transmitted torque consists of a handle to introduce a torque to the device, which is borne in the hollow of the handle and a driver-shaft as outpout element of the torque. On the driver-shaft inside the handle two separate couplings are assembled, between them a compression spring exerting a spring force onto the two couplings. Each coupling consist of a first and second socket engaging each other by gearings at their opposing faces. The maximum torque at which the torque-transmission will be interrupted cab be adjusted in that way, that by an adjustment-element the distance between the couplings can be changed resulting in a change of the tension of the spring exerting a load on the couplings. The teeth of the gearings slide and the sockets rotate relatively to each other and interrupt the torque-transmission if a pre-adjusted torque is exceeded.

BACKGROUND OF THE INVENTION

The Invention concerns a screwing-tool/screwdriver with a device to adjust a pre-selected maximum torque at which the transmission of torque from the handle to a screwdriver-shank will be interrupted.

Screwing-Tools with a device of that kind are known in different systems, in application at hand screwdrivers and power tools.

Of special interest under the aspect of the invention are such systems at which the torque is transmitted from hand of the user through the handle of the tool to the shank of a screwdriver-blade. Systems which have to be considered in relation to the system according invention are described in their construction and function as follows:

A screwdriver according U.S. Pat. No.5,746,298 with an adjustable torque limiting device is complicated in the construction of the torque transmitting coupling, which is made of metal. In the handle a rotateable shank is borne of which one end section is hexagon shaped in its cross-section. This end section is encompassed by six finger-shaped longitudinal end sections of a metal-sleeve which can turn relatively to the shaft. Each of the finger-shaped end sections contacts one of the six flats of the six flats of the hexagon shaft-section and apply a spring-force onto it. The finger shaped end sections are encompassed by a collar-shaped element which is adjustable in longitudinal direction and applies a radial force onto the finger shaped end sections pressing them onto the flats of the hexagon shaft section.

The closer the collar shaped element stands to the end of the finger shaped end section the stronger is the radial spring force applied onto the hexagon shaft section and the higher is the torque transmitted from the handle into the shank before the shank rotates relatively to the sleeve with the finger shaped end section respectively the handle and by this interrupts the torque transmission. The production of this torque limiting device is expensive in its production. To adjust the desired torque-limit the handle has to be disassembled partly. This system therefore is not practical for use when different torque-values have to be adjusted.

By DE OS 26 47 996 a system is disclosed which essentially consists of axial ball—and roller-bearings which are under axial load by a compression-spring. The balls and rollers are seated between discs and are taken in cavities.

One disc is connected with the torque-input-element the other disc with the torque-output-element of the device.

If the torque exceeds a particular adjusted value the force in circumferical direction will be higher than the axial force exerted by the spring and holding the balls and rollers in their seats the balls and rollers come out of their seats and the discs rotate relatively to each other interrupting the transmission of the torque.

By DE OS 2427 352 a system is described at which the torque transmitting device consist of a first shank with a disc-shaped end section as a coupling element. Its front face has in circumferical direction a wave-shaped profile. A second shank has also a disk-shaped end-section as coupling element with the same profile engaging with the one of the first shank.

By a compression of a pack of plate-ring-springs? an axial force is exerted on the first shank. If the torque is higher than the friction force between the two engaging coupling elements they rotate relatively to each other and the torque transmission is interrupted.

From DE OS 101 43 181 A1 a screwdriver with a device to limit a transmitted torque is known. The coupling between the handle and the screwdriver-shank consists of a tube-like metal part into which longitudinal short slots are cut. Small balls are sitting between disk-like coupling elements into which cavities are formed. The disk-like elements are under axial load of a compression-spring. With a part of their bodies the balls reach into the slots of the shaft-tube. If the torque exceeds the adjusted value given by the pressure of the spring the balls come out of their seats and are pressed into the slots in the shaft-tube and the torque transmission in interrupted. The construction basically corresponds to the construction as described by DE OS 26 47 996.

The device according DE OS 101 43 181 A1 is expensive in its production because is consists of metal parts of high precision.

At a screwdriver with a torque-limiting device as described in EP 1 092 510 A2 a shaft is borne in a handle, said shaft has a spherical end surface to transmit an axial force onto the inner end-surface of the hollow in the handle. The end section of the shaft is mushroom-like enlarged. The enlarged end section at its side opposite to its end surface is provided with a gear-profile in circumferical direction. This gear-profile engages into a gear profile of same shape which is formed into the rear end surface of a socket which is assembled axially moveable and rotateable onto the shaft. This socket is profiled at its circumferical surface with longitudinal flutes into which engage gear-like ribs standing radically off from the surface of the hollow in the handle. Through these ribs a torque is transmitted from the handle into the socket. A compression spring is assembled in the front section of the hollow and exerts an axial force onto the socket. The flanks of the teeth of the gear by which the socket and the enlarged-end section of the shaft engage are inclined in one circumferical direction and are standing perpendicular to the longitudinal axis of the system in the other longitudinal direction. If a torque load is applied which is higher than torque determined by adjusting a particular spring force in the direction of the inclined flanks the teeth of the socket will slide at the flanks of the gear profile in the mushroom-shaped end of the shaft, lift out of the gear profile and rotate relatively to each other. By this the torque transmission from the handle to the shaft will be interrupted.

The disadvantage of this system is that the axial force of the spring is lead through the end-surface of the shaft to the end surface of the hollow in the handle. This axial force is super-imposed by the axial force which is lead into the handle from the hand of a user. As consequence there will be a relatively high friction between the end surface of the shaft and the end surface of the hollow. The torque-value at which the transmission of torque will be interrupted is not only influenced by the axial force of the spring but also by axial force of the hand respectively the friction between the end surface of the shaft and the end surface of the hollow in the handle. The desired torque limit will therefore not be kept within desired tolerances.

Of advantage is that the essential part of the device are produced of plastic material by injection moulding. Of disadvantage is, however, that only relatively low torque can be transmitted. At higher torque plastical deformation at important parts occur, the functional precision of the system will be even less, possible not acceptable any more.

Some Torque Limiting Devices as shortly described before are relatively expensive in the production because they are complicated in their construction and consist of metal parts formed by cutting, turning, milling, drilling etc. The adjustment of a particular torque can not be done easily or the functional precision is not satisfactory.

Nowadays the demand for screwdrivers with a device allowing to adjust easily a limit for the transmitted torque has increased. Applications for such a tool are the fixing of tungsten-carbide cutting plates at machine tools, the assembling of electronic components or plastic parts. Corresponding with the increasing demand of this kind of tools is the demand for lower prices.

The task is to develop a tool of this kind which can be produced at relatively low cost, which can be used at torque loads covering the most applications at which such a tool can be used, at which the desired torque can be adjusted or set easily, keeping high precision in its function during a long time of use. This task is solved by the subject of the invention characterized by the claims.

SHORT DESCRIPTION OF THE INVENTION

The essential elements/parts of the object of the present invention are:

A handle for input of the torque, a screwdriver-shank for output of the torque, being transmitted from the handle to the shank through a torque limiting device.

Said device consists of two couplings assembled on a tube-like driver-shaft provided with teeth/a gearing on its surface extending over its length, between said couplings a compression-spring is arranged exerting an axial force onto the couplings. Each coupling consists of two cylindrical sockets/bushes engaging each other by teeth/a gearing at opposing sides. The first socket/bush is connected with the handle by teeth on its surface. In its boring the driver-shaft can rotate contacting by the ridge of the teeth the surface of the boring. The second socked/bush is connected with the driver-shaft by a radial gearing in its boring with play so that an axial movement is possible.

By the force exerted by the compression-spring the teeth of the first and second socket are engaging each other until the torque transmitted from the handle through the first sockets is higher than the friction between the teethed front faces of the first and second sockets, the teeth will slide relatively to each other forcing the second sockets to move axially against the spring-force, by this movement the transmission of the torque from the handle to the driver-shaft will be interrupted. The profile of the teeth at the front side of the first and second sockets/is constructed in that way that the flanks of the teeth in direction of fastening screws are inclined under an angle α, the edges of the teeth are slightly rounded and the ridge of the teeth is flattened or also rounded. The flanks of the teeth in direction of unscrew/loosen screws stand under an angle of 0° relative to a radial plane including the longitudinal axis of the device. In this direction no defined torque has to be adjusted or kept, further at an flank-angle of 0° it will be avoided that the shaft—and the screw—will be turned in unscrew-direction again when the teeth of two sockets/bushes come into grip-position again after they have rotated relatively to each other.

To adjust/set the desired torque-limit the rear coupling will be moved axially through an adjustment element, which is in contact with its front side with the rear side of the second socket and can be turned from outside the rear end or the cap of the handle by means of a screwdriver or key. The front side of the adjustment element is constructed as a collar/flange with a spiralic-like inclined/pitched ring surface, the same shape has the rear front side of second socket, the two inclined ring surfaces are forming a contact surface. By turning the adjustment element through the spiralic contact surfaces the distance of the rear coupling from the front coupling can be changed, by this the pre-tension of the spring between the two couplings. As consequence by this the value of the friction between the teeth of the first and second socket will be changed. Between the flange of the adjustment element and the inner surface of the cap an adjustment-disc is arranged which is teethed/geared at its front side, it also has teeth at its circumferical surface and by these teeth is unturneable linked with the handle. The rear side of the flange of the adjustment element is teethed/geared, as the adjustment disc. Both teethed surfaces are pressed onto each other by the force of the spring. By the pressure the rest-position of the adjustment element in circumferical direction is kept as adjusted when the adjustment element is turned. The adjustment element is provided with a pin at its rear side, the pin is borne in a boring of the cap of the handle and reaches to the outer surface of the cap. On the end surface of the pin there are scale-marks corresponding with a scale marked in the surface of the cap. The position of the scale-mark in the end surface of the pin to the scale in the surface of the cap indicates the value of the adjusted torque.

The driver-shaft is provided with a hollow in longitudinal direction with a hexagon cross section profile to take exchangeable screwdriver—or nut-setter-blades with hexagon shank. In a preferred embodiment the screwdriver-shank extends as far as near to the end of the driver-shaft. In another embodiment the driver shaft is provided with a chuck to take exchangeable tool inserts.

By turning of the driver-shaft the screwdriver-blade is also turned. The rear end of the teeth-shaft is support axially in a boring in the adjustment-element. As a result of this construction in use of the tool the force axially exerted from the hand of the user onto the handle and the cap is transmitted through the adjustment-disc and the adjustment-element onto the driver-shaft. The compression spring is not loaded by any hand force, therefore the tension of the spring is not influenced and has an effect only on the two couplings however not on the axial bearing of the teeth-shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

The object of the present invention is more precisely examplaryly described together with the drawings. There are shown by:

FIG. 1 is a longitudinal section through a screwing-tool with the device to limit the transmitted torque.

FIG. 2 is a second longitudinal section, in the lower half reaching as far as to the shank-end inserted into the driver-shaft.

FIG. 3 is a cross-section along line 1-1 in FIG. 1 through the clamping-device at the driver shaft, without inserted shank.

FIG. 4 is a cross-section along line 2-2 of FIG. 1 without shank.

FIG. 5 is a cross-section along line 3-3 in FIG. 1 without shank.

FIG. 6 is a cross-section along line 4-4 in FIG. 1 without shank.

FIG. 7 is a cross-section along line 5-5 in FIG. 1

FIG. 8 is a view from the rear side on the cap of the handle.

FIG. 9 is a side elevation view on the first (4) and second (3 a) socket of the first coupling and first (3 b) and second (5) socket of the second coupling.

FIG. 10 is a side elevation view on the first (4) and second (3 a) socket of the First coupling and first (3 b) and second (5) socket of the second coupling with a variation of the teeth-profile.

FIG. 11 is a longitudinal section showing the rear coupling displaced in direction to the coupling in the front part of the handle and the spring (9) more compressed.

FIG. 12 is an enlarged illustration of a section of the teething/gearing at the engaging faces of the first and second socket of the couplings.

FIG. 13 is a longitudinal section through the screwing-tool showing an embodiment of the handle with a coat of soft plastic material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODITMENTS

As FIG. 1 and FIG. 2 show the screwing-tool according the invention consists of a shank (1) with a non-circular cross-section profile, for example a hexagon profile, seated in the driver-shaft (2) and extending up to near the end of the teeth-shaft said shank (1) may be part of a screwdriver blade or of a holding device for screwdriver bits or for sockets. The driver-shaft (2) is profiled at it's circumference with a gear (2 a) over almost the full length, the teeth having preferably a rectangular profile. The first sockets (3 a, 3 b) are assembled on the driver-shaft (2). With their cylindrical surface the first sockets (3 a, 3 b) serve also as radial bearing element of the whole torque-limiting mechanism by contacting rotateably the wall of the hollow in the handle (14), said hollow being shut by the cap (20). With an inner radial gear in their boring (3 d in FIG. 5) the first sockets engage into the gearing (21) of the driver-shaft (2). At one front-side the first sockets (3 a, 3 b) have a gearing (21) with a wave-like teeth-profile at which the teeth are rounded at the tips. The gearings (21) gear in corresponding gearing of the second sockets (4, 5). At the other front-side the second socket (4) has a gearing (4 a) with rectangular profile which engages in a gearing with the same profile in the bottom of the hollow in the handle (14). A compression spring (9) is assembled with pre-tension between the two couplings (3 a, 3 b), resting on two washers (13). The second socket (5) is shaped with spiral pitch end surface (12) at the side opposite to the gearing (21). This surface (12) is in contact with a collar/flange of same shape formed at the adjustment element (6). At its circumference-surface this second socket has a gearing (5 a) with which it engages in a corresponding gearing (15) formed in the wall of the hollow of the handle (14). The adjustment-element (6) is provided with a fine gearing (6 a) at the side opposite to the collar/flange (12), also a pin (7) is formed at the adjustment-element. Said pin is borne in borings in the adjustment-disk (8) and the cap (20) of the handle. A recess (11) is moulded into the end of the pin (7) to allow to use a hexagon key—at the illustrated example—for turning the adjustment-element (6). Between the adjustment-element (6) and the cap (20) and adjustment-disc (8) is arranged. The adjustment-disc (8) is provided with a rectangular gearing (8 a) at its circumference surface engaging unturneable in a corresponding gearing in the hollow of the handle (14). A fine gearing (6 a) of the adjustment-disk (8) engages with a corresponding gearing formed into the rear side of the adjustment-element (6).

If the adjustment-element (6) is turned the collar/flange (12) with spiral pitch contacting the second socket (5) this socket together with the first socket (3 b) is moved axially and by this changing the pre-tension of the compression-spring (9). If a torque is applied on the handle (14) it will be transmitted by the gearings (4 a, 15) into the second sockets (4, 5) and further by the gearings (21) into the first sockets (3 a, 3 b) from said first sockets into the driver-shaft (2) and further into the shank (1).

By the pre-tension of the compression-spring it is determined up to which torque the gearing (21) of the first sockets (3 a, 3 b) and the second sockets (4, 5) will remain engaged until by rotation of the first sockets and the second sockets relatively to each other the transmission of the torque to the shank (1) will be interrupted.

A clamping device (18) at the front end of the driver-shaft (2) holds the shank (1) inserted into the driver-shaft by spring force exerted by an elastic tongue-like or saddle-like element (19) formed into the driver-shaft. At another embodiment the clamping-device consists of a band-ring-spring encompassing the front end of the driver-shaft (2). In a radial boring in the front end of the driver-shaft a small steel ball is borne which at one side protrudes into the hollow (2 b) of the driver shaft, at the other side protrudes out of the outer surface of the driver-shaft. A radial force is exerted by the band-ring-spring on the steel ball. If the shank is inserted into the hallow the steel ball is displaced radially outward and exerts a clamping force onto the shank.

FIG. 2 is a similar longitudinal section as FIG. 1, however the driver-shaft (2) in the lower half of the section is cut until its end and it may be seen that the inserted shank (1) extends to near the end of the driver-shaft.

FIG. 3 shows a cross-section along line 1-1 in FIG. 1 only through a clamping-device (18, 19) of the embodiment with the shank inserted exchangeable in the driver-shaft. The shank is borne unrotably in a pre-moulded hollow extending longitudinally in the driver-shaft and held by a clamping (18)-device at the front end of the driver-shaft (2). The clamping-device (18) is moulded as cylindrical part of the driver-shaft with a tongue-like or saddle-like elastic element (19) reaching with its lower end into he hollow (2 b) of the driver-shaft and being declined radially outward when the shank is inserted into the hollow. The spring-force of the declined elastic element holds the shank in its longitudinal position. At another embodiment the clamping-device consists of a band-ring-spring encompassing the front end of the driver-shaft (2). In a radial boring in the front end of the driver-shaft a small steel ball is borne which at one side protrudes into the hollow (2 b) of the driver-shaft, at the other side protrudes out of the outer surface of the driver-shaft. A radial force is exerted by the band-ring-spring on the steel ball. If the shank is inserted into the hollow the steel ball is displaced radially outward and exerts a clamping force onto the shank.

FIG. 4 shows a cross-section along line 2-2 in FIG. 1. As it can be seen the gearing (4 a) of the second socket (4) engages with a corresponding gearing in the handle (14) and connects both parts in that way that they can not rotate relatively to each other. The driver-shaft (2) with its gearing (2 a) is borne rotateably in the second socket (4). The hollow (2 b) to take the hexagon shank (1) also can be seen.

FIG. 5 is showing a cross-section along line 3-3 in FIG. 1. It can be seen that the first socket (3 b) is borne rotateably by its cylindrical circumference surface in the hollow of handle (14), also that the first socket (3 a) is linked by an inner gearing (3 d) with the gearing (2 a) of the driver-shaft (2). By the same way the other first socket (3 a) is also linked with the driver-shaft (2) and borne in the handle.

FIG. 6 shows a cross-section along line 4-4 in FIG. 1. The second socket (5) is linked by its gearing (5 a) with the gearing (15) of the handle (14). The driver-shaft (2) is borne rotateably in the second socket (5).

FIG. 7 shows a cross-section along line 5-5 in FIG. 1. The adjustment-disc (8) is engaging by the gearing (8 a) with the gearing (15) and by this firmly unturnably linked with the handle (14). The pin (7) of the adjustment-element (6) having the recess (11) is borne rotateably in the adjustment-disc (8).

By FIG. 8 scale-marks (16) at the pin (7) and the cap (20) are shown as a principle of the marking. For use it may be suitable to fix at the cap a scale-disc marked with the torque-value figures which can be adjusted.

As explained before the range of adjustable torque values depends from the spring-characteristic of the spring (9) inserted in the device, and this range will be measured by calibrating the torque-adjusting-system. The construction of the adjustment-element (6) and the second socket (5) with spiralic pitched engaging/contacting surface (12) allow to achieve a very precise calibration. The pitch of the surfaces (12) at the second socket (5) and at the adjustment-element (6) determines how much the transmittable torque will change at a particular angle of turn at the adjustment-element (6). If the pitch is low the second socket (5) and the first socket (3 b) will be moved axially only by a small distance at a particular angle of turn of the adjustment-element. Consequently the length of the compression-spring (9) will change only by this small distance and correspondingly the pre-tension of the spring and the transmittable torque will change only slightly.

FIGS. 9 and 10 show the couplings with different shapes of the gear (21 a, 21 b). By the shape of the tooth the torque at which the teeth start to slide at the flanks and disengage can be influenced. If the angle a of the flank is large relative to the longitudinal axis of the coupling, the teeth will start to slide at a lower torque than if a is small. If the angle a is 0°, as shown in FIG. 10 for a left-turn-direction, there will be no disengagement of the coupling at all. At this angle it will be avoided also that a screw will be turned in direction to loosen/unscrew again if the resistance of the screw to turn is low.

In FIG. 11, a longitudinal section through the screwing-tool it is shown that the first and second socket (3 b, 5) of the second/rear coupling are displaced forward in direction to the first coupling by turning the adjustment-element (6). By this the compression-spring (9) has been more compressed, the wide gap between the low area of the spiral-pitched collar/flange of the adjustment-element (6) and the spiral-pitched end surface (12) of the second socket (5) is obvious. By turning the adjustment-element (6) by means of a key inserted into the recess in the pin a pre-determined torque can be set to be transmitted into the shank (1). By calibrating the torque-transmitting device the values of adjustable torque can be found and marked on scales (6, 7) at the end of the handle. By selecting the compression-spring (9) with a particular spring characteristic the desired adjustable torque-values can be determined. It is also possible to cover different ranges of adjustable torque-values by removing the cap (20) from the handle (14), exchanging the compression-spring (9) by a spring with another spring-characteristic and re-assembling the device and cap. The adjustment-element (6) serves also as bearing for the end (10) of the driver-shaft (2), especially as axial bearing as shown in FIG. 2. Axial forces are transmitted from the adjustment-element (6) through the adjustment-disc (8) into the cap (20) or respectively will be transmitted in the other direction in use of the screwdriver from the hand of the user into the handle (14) and the cap (20) into the shank (1). At another embodiment the shank (1) is firmly inserted into the driver-shaft (2).

FIG. 12 shows in a larger scale the preferred wave-like shape of the teeth of the gearing (21) with which the first and second sockets of the two couplings are engaging each other. In FIG. 13 an embodiment of the handle (14) is illustrated which has a coat (22) of soft plastic material injection molded onto the core-handle made of hard plastic material.

The construction of the device with two couplings and the other elements according the present invention allow to transmit higher torque than by known devices because the specific loads on the different torque-transmitting elements are much lower than by a device with one coupling only. This is achieved by introducing torque at two areas namely through the gearing (4 a) at the front side face of the second socket (4) and through the gearing (5 a) at the circumferical surface of the second socket (5) respectively the inner gearing (15) of the handle (14) and transmit torque to the driver-shaft (2) also through two elements namely two first sockets (3 a, 3 b). The lower specific loads at the torque transmitting areas or elements allow to produce all the parts/elements of the device—with exception of the shaft (1)—the spring (9) and the washers (13) of plastic material by injection-molding. Polyamide (PA) and Polyoxymethylene (POM) have been found to be suitable plastic materials, especially for those elements/parts which are sliding at each other. Of special advantage is a composition of POM with Teflon or a sliding improving additive to the basic material. Using this material the friction between sliding surfaces is reduced remarkably which is of special importance at the gearing (21) of the first sockets (3 a, 3 b) engaged with the gearing of the second sockets (4, 5). By reduction of the friction the influence of the friction on the transmitted torque is reduced which further improves the precision in function of the device. Screwdrivers with a device to adjust a transmitted pre-selected torque are used also in the orthopedic surgery. It is necessary to sterilize the instruments after use. In dry-air sterilizing apparatus a temperature of 180° C. will be reached. No deformation of parts of the torque-adjusting device is allowed at this temperature.

In a special execution the screwing tool is made of plastic material which has a heat resistance and shape stability up to 220° C. Particular parts as the teeth-shaft (2), the first and second sockets (3 a, 3 b, 4, 5), the adjustment element (6) and the adjustment disc (8) or only some of these parts may alternatively be made of metal, for example as parts sintered of powder-metal of a quality or metal-pairing which have a good sliding-characteristic. The parts may also be made of steel formed by turning, milling or pressing. Also the handle (14) and the cap (20) may be made of metal, for example of aluminium as cast-parts. Finally the handle, the cap and other parts of the device, especially those which have to withstand a static load during the use of the tool may be made of stainless steel. The metal-execution may be considered if with dimension as at the plastic-material-execution higher torque has to be transmitted or for use in the orthopedic surchery the tool has to withstand sterilization's without any limit. As a metal-handle will be designed considering the requirement of durability and not so much the requirements of ergonomy—to save weight—at a variation of the embodiment of the core handle with uncircular outer cross-section contour receives a sleeve preferably made of elastic plastic material. The outer contour of this plastic sleeve is shaped to meet the requirements of ergonomy. The sleeve may be withdrawn from the score handle and disposed after use of the tool. This embodiment is illustrated in FIG. 13.

SUMMARY OF ADVANTAGES

Of special advantages is the tool according the present invention with respect to the adjustment of the torque. The spiralic pitched collar/flange (12) of the adjustment element (6) in contact with the spiralic pitched rear side face of the second socket (5) functioning together with the fine gearing (6 a) at the adjustment-disc (8) and at the adjustment-element (6) allow to adjust the pre-tension of the compression-spring (9) in very fine steps resulting in a very precise adjustment of a pre-selected torque at which the torque-transmission is interrupted. If the tool is designed to be used as calibrating-tool for other torque-measuring tools a low pitch-angle of the spiralic surfaces (12) will be chosen. Then the torque-values may be adjusted even more precisely.

By the arrangement of two couplings on the driver-shaft (2) the pre-tension of the spring (9) is distributed equally on both couplings, the contacting surface is increased and consequently the specific load on them reduced. The spring-force does not exert an axial load on the driver-shaft (2) and by this was avoiding an influence by friction in the axial bearing of the driver-shaft on the torque transmitted from the handle through the torque-adjusting and limiting device to the shank (1).

Of advantage is also that all parts of the tool according the present invention can easily be assembled. The tool can be produced at relatively low cost, it is functioning with high precision and has a long-lasting durability. 

1. A screwing tool with an adjustable device to limit transmitted torque, said device essentially consisting of a handle with an axially extending hollow in which a mechanism is arranged to interrupt the transmission of torque from the handle to a driver-shaft if a pre-adjusted torque-value is exceeded, said mechanism essentially consists of a driver-shaft on which two couplings are assembled, each coupling consisting of a first and second socket which are engaging each other by a gearing at their opposing side faces, both first sockets are engaging with an inner gearing with a gearing on the circumferical surface of the driver shaft and are moveable in longitudinal direction, both second sockets are unturneable in mesh with the handle by means of a gearing, and a compression-spring arranged with pre-tension between the two couplings, said hollow in the handle being shut by a cap.
 2. A screwing tool as defined in claim 1 wherein the first sockets are borne and radially support with their cylindrical surface by the cylindrical hollow in the handle which encloses the sockets at least at a part of their longitudinal extension.
 3. A screwing tool as defined in claim 1 wherein the second sockets have an axial boring in which the driver-shaft is borne rotateably.
 4. The screwing tool as defined in claim 1 wherein the second socket of the front coupling is unturneable engaged with the handle by a gearing at its front side.
 5. The screwing tool as defined in claim 1 wherein the second socket of the rear coupling is borne moveably in longitudinal (axial) direction on the driver-shaft, said second socket having a geared circumferical surface with which it is engaging in a gearing which is molded into this part of the hollow of the handle.
 6. The screwing tool as defined in claim 1 wherein the rear end face of the second socked of the rear coupling is shaped as a spiralic pitched surface.
 7. The screwing tool as defined in claim 1 wherein an adjustment element is arranged behind the second socket of the rear coupling, at this adjustment element the face opposite to the second socket of the rear coupling is also shaped as spiralic pitched surface with which it contacts the spiralic surface of the second socket of the rear coupling, said adjustment element at the rear face has a fine gearing.
 8. The screwing tool as defined in claim 1 wherein an adjustment-disc is arranged between the adjustment element and the cap of the handle, said adjustment-disc at its face opposite to the adjustment element also has a fine gearing engaging with the gearing of the adjustment element, both parts are pressed together by the force exerted by the compression spring.
 9. The screwing tool as defined in claim 8 wherein the adjustment element has a pin with which it penetrates the adjustment disc and the cap of the handle and is borne in these parts, the pin has a recess at its end provided for to insert a key to turn the adjustment element.
 10. The screwing tool as defined in claim 8 wherein the graduation of the gearing at the adjustment disc are so chosen that within a given range of torque values intermediate torque values can be adjusted in equal steps.
 11. The screwing tool as defined in claim 10 wherein the surface of the pin and the surface of the cap around the pin are marked with scale-marks and the position of the markings relative to each other indicate the pre-tension of the spring respectively the adjusted maximum torque which will be transmitted.
 12. The screwing tool as defined in claim 1 wherein the gearing of the first and second sockets has essentially a triangular tooth-profile at which the angle of the teeth-flanks relative to the axis of the sockets is larger in direction of fastening screws —a right-turn—than in direction of unscrewing—a left-turn.
 13. The screwing tool as defined in claim 1 wherein the angle a of the teeth-flanks relative to the axis of the sockets in direction of fastening screws is larger if the device is designed for a range of lower torques than if the device is designed for a range of higher torques.
 14. The screwing tool as defined in claim 14 wherein the angle of the teeth-flanks relative to the axis of the sockets is 0° in direction to unscrew screws.
 15. The screwing tool as defined in claim 14 wherein the profile of the teeth is shaped wave-like and the teeth are rounded or flattened at their ridge.
 16. The screwing tool as defined in claim 1 wherein the gearing at the driver-shaft, in the boring of the first sockets, at the circumferical surface of the second socket of the rear coupling and in the hollow of the handle have essentially a rectangular tooth-profile.
 17. The screwing tool as defined in claim 1 wherein the handle and the cap are connected by thread or in an alternative embodyment inseparably.
 18. The screwing tool as defined in claim 1 wherein the driver-shaft, the first and second sockets of the couplings, the adjustment-element, the adjustment-disc, the compression spring, the handle and the cap are arranged in that way that there is no axial load exerted onto the driver-shaft.
 19. The screwing tool as defined in claim 1 wherein the axial force exerted by the compression spring is lead in one direction through the first and second socket, the adjustment-element and the adjustment-disc into the cap of the handle, in the other direction through the first and second socket into the front part of the handle.
 20. The screwing tool as defined in claim 1 wherein the driver shaft, the first and second sockets, adjustment-element, the adjustment-disc, handle and cap are made by injection molding of plastic-materials suitable for the application at the tool.
 21. The screwing tool as defined in claim 1 wherein of parts with surfaces sliding at each other one part preferably is made of Polyamide (PA) and the other part preferably is made of Polyoxymethylene (POM) where as at least at one part of the basic material is compounded with Teflon or a sliding improving additive.
 22. The screwing tool as defined in claim 1 wherein the plastic materials used have a heat-resistance up to 220° C.
 23. The screwing tool as defined in claim 1 wherein the driver-shaft, the first and second sockets, the adjustment-element, the adjustment-disc, the handle, the cap or individuals of these parts are made of metal.
 24. The screwing tool as defined in claim 1 wherein a core-handle made of metal has an outer cross-section contour which is non-circular and an economically shaped handle-sleeve made of plastic material is exchangeably assembled on the metal handle.
 25. The screwing tool as defined in claim 1 wherein the shank of a screwdriver-blade or a holder for screwdriver-inserts (bits) is inserted exchangeably in the driver shaft and held its position by a clamping-device connected with the driver shaft. 